feat(api): convert from SD checkpoints

[ssube]

Convert SD checkpoints from .ckpt and .safetensors to ONNX.

• add a way to download files by putting https:// links in the extras.json file
• store downloaded files and torch models in a cache directory (maybe models/cache)
• fix/remove the requirement to have -onnx- in the model name
• make sure licenses are included/accurate for everything

Misc fixes:

• use correct device on CPU-only systems
• use consistent device for CUDA training (device error converting SD upscale model on CUDA #119)

Follow up:

• add an option to remove temporary files and models after converting #122: add a way to remove torch models after conversion is done

[sonarcloud]

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